The new Air Force bomber will replace the B-2 (pictured) and B-52 fleets. (Senior Airman Kenny Holston/Air Force)

The Air Force expects to select its manufacturer for the next generation bomber in the spring, with contractors notified today of the service's requirements for the aircraft.

The Air Force released its request for proposal to industry for the long-range strike bomber Wednesday, the service said today in a statement.

“The LRS-B is a top modernization priority for the Air Force," Air Force Secretary Deborah Lee James said in the statement. "It will be an adaptable and highly capable system based upon mature technology. We look forward to industry's best efforts in supporting this critical national security capability."

The Air Force will not say when the proposals are due. The release puts the Air Force "officially in the competitive phase and greatly limits what we can say," spokesman Ed Gulick said.

Air Force and Defense Department officials have been tight-lipped on the schedule for this request, and any further details on the system.

The service expects to field 80 to 100 of the bombers to replace the aging B-52 Stratofortress and B-2 Spirit fleets.

Officials have said that it is aiming for a mid-2020s operational date, and that the aircraft needs room for a large payload. Service officials have also said they are exploring the possibility of the aircraft being optionally manned.

“That program is designed around a fixed set of requirements [with] relatively mature technologies, [where we will] build the first version knowing it won’t have everything on it that we want or will want,” said William LaPlante, the assistant Air Force secretary for acquisition, in a June speech. “We’re building an adaptable approach with an open architecture, [with] places on the wings that allow us to customize sensors and weapons with future capabilities.

The Air Force wants to keep the cost for the program under $550 million per plane.